Here is the thing about security, you want it simple. You need people to easily understand what they can and can't access. The Confidential, Secret, and Top Secret classifications do that. But at the same time, when your security classification is too broad, then people without a "Need to Know" can access items they don't need to within their security classification. The Jonathan Pollard case is a good example of this. Due to his style of espionage, things like SCI were invented that further restrict sensitive items to only those who have a "Need to Know". Now your security system is complicated. Who do you give SCI tickets to and how do you track them and adjust over time? There is no simple solution. SCI complicates security and makes it difficult to work or share work with people that can help you out. It prevents people from knowing the Big Picture. But it also prevents assholes from selling major national security secrets to foreign countries.